1. Field of the Invention
The invention relates to a method for the manufacture of a ceramic membrane for micro-filtration comprising a porous ceramic carrier and a micro-filtration coating of sintered ceramic material, on a bulk surface thereof. By "bulk surface" is meant a bounding surface or geometric surface of the carrier body, not the interior pore surfaces thereof.
2. Description of the Prior Art
A micro-filtration coating in such a membrane typically is a coating with an average pore size of between 20 nm and 1 .mu.m and a thickness of between 10 and 200 .mu.m. The carrier may have pores of between 0.1 and 50 .mu.m and may have a thickness of between 0.5 mm and 1 cm or greater. These dimensions are not limitative for the present invention.
EP-A-320033 describes a process for making such a ceramic membrane, comprising the steps of
(i) silanising the carrier; PA1 (ii) applying the micro-filtration coating onto the carrier by means of a film-coating technique using a suspension of a metal oxide; PA1 (iii) drying and sintering the product obtained in step (ii). PA1 (a) A stable suspension can be prepared in a reproducible way. The properties of the stable suspension, such as particle size distribution and yield stress are only slightly susceptible to small differences in pH, quantity of additives and intensity of stirring and mixing; PA1 (b) The suspension is only slightly susceptible to ageing. The suspension properties barely alter with time; PA1 (c) The reproducibility of the micro-filtration coating obtained by film-coating is good; PA1 (d) The packing and the pore size distribution of the micro-filtration coating is homogeneous. This means that far fewer cracks occur in the micro-filtration coating; PA1 (e) A relatively thicker micro-filtration coating can be obtained without cracks forming.
Silanisation is a pretreatment of the carrier, described for example in EP-A-320033, in which the affinity between the porous carrier and the suspension forming the microporous coating is reduced so that the suspended particles are prevented from penetrating the pores of the carrier, while it is still possible sufficiently to wet the surface of the porous carrier. Reduction of this affinity is preferably obtained by treating the carrier with a silane solution.
Film-coating is a method for applying the coating onto the carrier by essentially allowing a suspension to flow along the surface of the carrier, e.g. under the effect of gravity, so that as it flows a coating film remains on the bulk surface of the carrier. EP-A-320033 mentioned above illustrates film coating. Further explanation may be found in J. A. Tallmadge & Ch. Gutfinger: `Entrainment of liquid films`, Ind. Eng. Chem., 59(11), 18-34 (1967) and L. E. Scriven: `Physics and application of dip coating and spin coating`, in Better ceramics through chemistry, III (1988), pp. 717-729.
Compared with this film-coating, another method exists for applying the micro-filtration coating, namely slip casting, in which a suspension is filtered through the porous carrier under the effect of a pressure difference, so that a coating remains on the carrier as filter cake. This slip casting technique has the disadvantage that particles from the suspension penetrate the pores of the carrier, so that the permeability of the resulting membrane is lower.
A problem with the method known from EP-A-320033 is that in the micro-filtration coating an inhomogeneous or broad pore size distribution is obtained so that shrinkage is non-uniform and sintering cracks arise in the sintered micro-filtration coating. This gives a high reject percentage of membranes obtained with the known method.
With hindsight, it can be seen that in EP-A-320033, insufficient attention has been paid to the condition of the suspension when it contacts the silanised porous ceramic carrier. In order to obtain a thick layer, in Examples 1 and 2 of that document the suspension used contained alumina dispersed in demineralised water. At the pH of this suspension, alumina particles tend to agglomerate, with the result that relatively large particles in the form of agglomerates are deposited on the carrier surface. When the deposited layer is subsequently sintered, the layer shows non-uniform behaviour because of the presence of the agglomerates. The sintering is therefore inhomogeneous, leading to an inhomogeneous product, with cracks due to non-uniform shrinkage. On the other hand, in Example 3 of EP-A-320033, a peptised sol is used to produce a very thin layer. The peptised sol contains primary particles, but not agglomerates. This sol also contains 1% of glycerol. It is mentioned here that glycerol has no effect on the yield stress of the sol.
EP-A-344961 describes a similar method of forming a composite membrane useful for filtration having a metal carrier and a ceramic coating. In Example 1, there are described some experiments to determine an optimum viscosity of a suspension (specifically a boehmite sol) which is used to coat stainless steel membranes. The steel membrane is pretreated with a solution of sodium dichromate and sulphuric acid. To vary the viscosity of the boehmite sol, the binders polyvinyl alcohol and methyl cellulose were added. The stated aim was to achieve an appreciable increase in viscosity, with the purpose of aiding the sol or suspension to bridge the rather coarse pores of the support. It was found that the minimum binder addition was 10%, in order to yield an appreciable increase in viscosity. More than 50 wt % of binder results in gelation. Optimum binder additions were determined to be about 18 wt % for one particular sol and about 40 wt % for a sol of lower concentration. The sols were deposited by spray coating. Multiple spraying with up to five coats was necessary to ensure a complete coating. The maximum film thickness obtained was 10 .mu.m. Although addition of binders in such large concentrations in order to increase viscosity may also lead to an increase of the yield stress of the suspension or sol, there is no discussion of this, and it is not relevant to the process described in this document which involves spray coating. A disadvantage of such high concentrations of binder is that the large amount of binder must be burned out of the coating layer prior to the sintering, which causes defects in the layer such as large pores, peeling of the layer or crater formation.
In the present specification, the term suspension is used, and is intended to include also sols and colloids. The difference between these various forms of suspension is only a matter of particle size.